▎ 摘　　要

Based on first-principles calculations and semiclassical transport theory, we study the nonequilibrium phonon distribution in graphene in the presence of electrical current. In addition to the hot-phonon effect, we observe a cooling of phonon modes at the same time. Interestingly, the presence of electric current along the direction connecting K and K' valleys induces an opposite dipolelike temperature distribution in the two valleys and at the gamma point. This leads to a "high-order " valley polarization of phonon distribution between K and K'. Based on the nonequilibrium phonon distribution, we furthermore study their effect on the lattice parameters and find a negative current-induced expansion coefficient. Similar to graphene's negative thermal expansion, this is rooted in the dominant contribution of out-of-plane acoustic phonons, which do not couple directly to electrons, but gain energy from in-plane phonons through anharmonic scattering.